1. Field of the Invention
The present invention relates to a light emitting element and a method for driving a light emitting device manufactured by forming thin-film transistors (hereinafter abbreviated as TFTs) on a substrate. Further, the present invention relates to electronic apparatuses using the light-emitting device as a display unit.
Within this specification, as a representative example of the light emitting element, an Electro Luminescence (EL) element will be used. Further, the EL element includes the ones which utilize emission of light from singlet excitons (fluorescence) and the ones which utilize the emission of light from triplet excitons (phosphorescence).
2. Description of the Related Art
In recent years, light-emitting devices having EL elements have been vigorously developed as self light emitting elements. Unlike the liquid crystal display devices, the light-emitting device is of self light emitting type. The EL element has a structure in which an EL layer is held between a pair of electrodes (anode and cathode), the EL layer being, usually, of a laminated-layer structure. Typically, there can be exemplified a laminated-layer structure of “positive hole-transporting layer/light-emitting layer/electron-transporting layer”. This structure features a very high light-emitting efficiency, and the EL display devices that have now been studied and developed have almost all been employing this structure.
There can be further exemplified a structure in which a positive hole-injecting layer, a positive hole-transporting layer, a light-emitting layer and an electron-transporting layer are laminated in this order on the anode, or a structure in which the positive hole-injecting layer, the positive hole-transporting layer, the light-emitting layer, the electron-transporting layer and the electron-injecting layer are laminated thereon this order. The light-emitting layer may further be doped with a fluorescent pigment or the like pigment.
In this specification, the layers provided between the cathode and the anode are all referred generally as an EL layer. Therefore, the above positive hole-injecting layer, positive hole-transporting layer, light-emitting layer, electron-transporting layer and electron-injection layer are all included in the EL layer.
A predetermined voltage is applied across the pair of electrodes (both electrodes) holding the EL layer of the above structure therein, whereby the carriers are recombined in the light-emitting layer to thereby emit light. At this time, the luminance of the EL element is in proportion to a current flowing to the EL element.
The light-emitting devices can include those of the passive matrix type and those of the active matrix type. Here, the devices of the active matrix type are suited for the applications where a high-speed operation is required for the increase in the number of pixels accompanying high resolution and displaying moving images.
As a method of driving an EL element, there are a method of driving at a constant voltage, in which a certain voltage is applied to the EL element and a method of driving at a constant current, in which a certain electric current is flowed to the EL element. In the method of driving at a constant voltage, the electric resistance of an EL element changes depending upon the temperature variation and the amount of electric current flowing to the EL element also changes. Moreover, the electric resistance of the EL element increases, and the amount of electric current flowing to the EL element decreases, due to the changes over time. Since the brightness of the EL element is in proportion to the electric current, the brightness also changes along with those of the electric current. Hence, as a method of driving an EL element, it may be desirable to employ the method of driving at a constant electric current.
However, besides the change of electric resistance of the EL element, such deterioration occurs as the brightness of the EL element being lowered due to the changes over time, even if a certain electric current is flowed. Particularly, the lowering of the brightness due to the deterioration of the EL element at the time of initial lighting, which is called an “initial deterioration,” is significant.
Hence, for the purpose of suppressing the deterioration of the EL element and enhancing the reliability, there is a method of applying the reverse bias to the EL element.
As for a reverse bias application voltage, it has been disclosed in JP-A-08-180972 gazette, that preferably the reverse bias voltage is made larger than or equal to the forward bias voltage.
Moreover, as for a method of driving an active matrix type light emitting device, there is a digital time gradation method which is not easily influenced by the variation of the characteristics of the TFT for driving. That is a method in which each pixel is configured with two transistors, a TFT for driving and a TFT for switching, one frame period is divided into an address (writing) period and a sustain (light emitting) period, and the gradation is controlled by the sum of the lengths of the time for emitting the light.
Moreover, there is a digital time gradation method in which poly-gradation display with a high precision can be realized by utilizing three transistors, specifically, a TFT for driving, a TFT for switching and a TFT for blanking. In the present specification, a digital time gradation method using these three transistors for each pixel is defined as SES (Simultaneous Erase Scan) drive. It should be noted that concerning with this SES drive, the detail of it has been disclosed in JP-A-2001-343933 gazette.
It is contemplated that the reverse bias is applied by the SES drive at the time when an EL element is driven at a constant electric current. In an active matrix type light emitting device, in order to drive the EL element at the constant electric current, the TFT for driving and the EL element are put in series and in order to operate the TFT for driving in the saturation region, a high voltage is required.
Furthermore, in the case where the reverse bias voltage is made larger than the forward bias voltage, if an electrode in which the capacitance loading is large is changed, such a problem occurs as increase of the consumption of the electric power. Moreover, it is considered that malfunctions indicated in the followings may occur depending upon the reverse bias application method. The pixel configuration in the case where the foregoing SES drive method is performed is shown in FIG. 2. Moreover, the reason will be described with reference to FIG. 2.
Each pixel has a source signal line 201, a gate signal line for writing 202, a gate signal line for blanking 203, a TFT for switching 204, a TFT for blanking 205, a TFT for driving 206, an EL element 207 and an electric current supplying line 208, one of EL elements 207 is connected to a source electrode of the TFT for driving 206 or a drain electrode (pixel electrode), and the other is connected to a counter electrode 209.
Here, the TFT for switching 204 and the TFT for blanking 205 are N-channel type TFTs, the TFT for driving 206 is a P-channel type TFT, and in the EL element 207, the side connected to the TFT for driving 206 is made an anode, the side connected to the counter electrode 209 is made a cathode. For the purpose of clarifying the description, each potential is shown within the parenthesis ( ) as an example. However, these potentials are solely examples, when it is driven by the following methods, it may be appropriately set at the desired potential.
First, a pulse (8V) by which the TFT for switching 204 is turned ON is inputted into the gate signal line for writing 202, the TFT for switching 204 is turned ON, and a picture signal outputted into the source signal line 201 is applied to a gate electrode of the TFT for driving 206. Here, since the TFT for driving 206 is a P-channel type TFT, when the picture signal is at H level (6 V), it is turned OFF, and when it is at L level (0 V), it is turned ON.
Subsequently, by turning the TFT for driving 207 ON, the electric current flows through the EL element 207 from the electric current supplying line 208 (5 V) towards the counter electrode 209, and the light emits. The TFT for driving 206 is operated in the saturation region. Moreover, when the TFT for driving 206 is turned OFF, the electric current does not flow into the EL element 207, it becomes in a non-light emitting state.
Subsequently, a pulse (8V) for turning the TFT for blanking ON is inputted into the gate signal line for blanking 203, and the TFT for blanking 205 is turned ON. The potential of the electric current supplying line 208 is inputted to the gate electrode of the TFT for driving 206 by turning the TFT for blanking 205 ON, the voltage between the gate and the source of the TFT for driving 206 becomes 0, the TFT for driving 206 is turned OFF. Therefore, the EL element 207 becomes in a non-light emitting state.
Here, the reverse bias period is provided in a non-light emitting period, and consider the case where the reverse bias is applied to the EL element 207. In the case where the reverse bias is applied by greatly changing the potential of the electric current supplying line 208, the potential at the time when the gate signal line for writing 202 is turned OFF must be increased.
For example, when the potential of the electric current supplying line 208 was greatly changed (5 V→−22 V), the TFT for blanking 205 is turned ON and the potential of the gate electrode of the TFT for driving 206 becomes the potential of the electric current supplying line 208 (−22 V) since the potential of the gate electrode of the TFT for blanking 205 (−2 V) is higher than the potential of the electric current supplying line 208 by the portion of more than the threshold of the TFT for blanking 205. Therefore, the TFT for driving 206 is also turned ON, the potential of the pixel electrode becomes the potential raised from the potential of the gate electrode of the TFT for driving 206 by the portion of the threshold voltage of the TFT for driving 206 (about −20 V). Therefore, as a result, the reverse bias voltage (about 10 V) is applied to the EL element.
However, when paying attention to the TFT for switching 204 at this time, the TFT for switching 204 is turned ON since the potential of the gate electrode (−2 V) is higher than the potential of the gate electrode of the TFT for driving 206 (about −20 V) by the portion of more than the threshold of the TFT for switching 204. Specifically, the electric current supplying line 208 and the source signal line 201 are shorted while sandwiching the TFT for switching 204, the TFT for blanking 205. In this way, in order to prevent the TFT which originally should not be turned ON from being turned ON, the potential at the time when the gate signal line for writing 202 must be further lowered (about −24 V). However, in this case, the increase of the consumption of electric power of the gate driver becomes a problem by increasing the voltage amplitude of the signal as well as the uncertainty is generated on the withstand voltage of the TFT.
Moreover, by changing greatly the reverse bias voltage, the other section which has capacitive coupling to the section for changing the voltage (electric current supplying line 208 and the like) is changed by the voltage, being influenced at the time when the reverse bias is applied. Due to this, it is also considered that the transistor which should be turned OFF is turned ON, or the consumption of electric power are increased by charging and discharging the moved electric charge and so forth.
Moreover, by increasing the reverse bias voltage (10V) more than the forward bias voltage (8V), the changes of the potential of the electric current supplying line 208 (27 V) further become larger, even if the deterioration of the EL element is suppressed, the demerit such as the increase of the consumption of electric power and the like cannot be avoided.
Hence, the present invention aims at suppressing the increase of the consumption of electric power and enhancing the reliability of the EL element, and proposes an alternate current drive method in which the reverse bias is applied to the EL element driving at the constant electric current in the SES drive.